Make GetAvailableCredit run GetHash() only once per transaction. #3973

This makes the first getbalance/getinfo 63x faster on my wallet.

Putting this up so other people complaining about slow getinfo can try it. I'd like to better understand exactly why it was taking 12.6 seconds for me in the first place, as it's a bit surprising that computing the transaction hashes would be that slow.

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ACK on the change.

ACK on the change.

Though a more general solution would be to store re-compute the transaction hash only on changes to a transaction. GetHash() is called a lot also in other contexts, if it is really this slow that may make sense.

From looking at the behavior we seem to be wasting just a ton of time in serialization code (and associated memory allocation) when computing GetHash... there is probably a lot of general room for improvement there, though regardless not calling it extra times is probably go no matter how much we improve it. Sounds like other people are reporting it solved their issues.